The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art.
The ability to transport fluids in micron-sized channels is important for many emerging technologies, such as in vivo drug delivery devices, micro-electro-mechanical systems (MEMS), and micro-total-analysis systems (μTAS). Common fluids used in microfluidic devices include whole blood samples, bacterial cell suspensions, protein or antibody solutions and various buffers. Microfluidic devices can be used to obtain a variety of measurements including molecular diffusion coefficients, fluid viscosity, pH, chemical binding coefficients and enzyme reaction kinetics. Other applications for microfluidic devices include capillary electrophoresis, isoelectric focusing, immunoassays, flow cytometry, sample injection of proteins for analysis via mass spectrometry, PCR amplification, DNA analysis, cell manipulation, cell separation, cell patterning and chemical gradient formation. Many of these applications have utility for clinical diagnostics.
The use of microfluidic devices to conduct pharmaceutical research and create clinically useful technologies has a number of significant advantages. First, because the volume of fluids within these channels is usually several microliters or nanoliters, the amount of reagents and analytes used is quite small. This is especially useful for expensive reagents. The fabrications techniques used to construct microfluidic devices are relatively inexpensive and are very amenable both to highly elaborate, multiplexed devices and also to mass production. In a manner similar to that for microelectronics, microfluidic technologies enable the fabrication of highly integrated devices for performing several different functions on the same substrate chip.
There are two common methods by which fluid actuation through microchannels can be achieved. In pressure driven flow, the fluid is pumped through the device via positive displacement pumps, such as syringe pumps. Another common technique for pumping fluids is that of electroosmotic pumping. If the walls of a microchannel have an electric charge, as most surfaces do, an electric double layer of counter ions will form at the walls. When an electric field is applied across the channel, the ions in the double layer move towards the electrode of opposite polarity. This creates motion of the fluid near the walls and transfers via viscous forces into convective motion of the bulk fluid. If the channel is open at the electrodes, the velocity profile is uniform across the entire width of the channel. Devices utilizing the electroosmotic effect are particularly applicable in microfluidics where the manipulation of small amounts of electrolyte solution is required to perform chemical or biochemical reactions.